The spine includes a series of joints or motion segments. The components of each motion segment include two adjacent vertebrae and their apophyseal joints, an intervertebral disc, and connecting ligamentous tissue. Each motion segment is capable of flexion, extension, lateral bending, and translation. Each component of the motion segment contributes to the mechanical stability of that joint. Overall, the motion segments enable the familiar kinematic motion of the spinal column.
Components of a motion segment that move out of position, become damaged, deteriorate, or are disordered in some fashion, can lead to serious pain. Furthermore, these disorders may lead to injury to other components of the spine. Depending upon the degree of disorder or damage that has occurred, treatment may include fusing the components of the motion segment together.
Fusion procedures include rigid stabilization of one or more motion segments, i.e., immobilization of the motion segment. Fusing usually involves fixation of a metallic rod, plate, or other rigid member to components of the disordered motion segment to promote fusion within and between these components. However, fusing motion segments may lead to other problems. Simply put, fusion results in a loss of mobility in that motion segment. The lost mobility in the fused motion segment transfers the required movement to other, non-fused motion segments. Adjacent, non-fused motion segments experience the greatest transfer of the demand for motion. Added demand increases the stress on the non-fused motion segments, and, consequently, causes non-fused motion segments to deteriorate. Therefore, loss of motion in one motion segment may contribute to or even cause disorders in the motion segments above and below the fused motion segment.
Thus, a stabilization member that permits limited motion of the disordered motion segment and which reduces the demand for motion on adjacent motion segments is needed.